During the past year we have completed a study on the B cell response of patients with chronic HCV infection in the context of mixed cryoglobulinemia, and the impact of rituximab treatment on B cell function. Mixed cryoglobulinemia (MC) is the most common extrahepatic manifestation of chronic hepatitis C virus (HCV) infection, observed in more than 50% of patients. Immune complexes consisting of HCV, anti-HCV IgG, rheumatoid factor (RF) IgM molecules and complement circulate in the blood and in a subgroup of patients, these complexes trigger inflammation in small blood vessels leading to life threatening conditions such as neuropathies and renal failure. Clonal expansions of autoreactive memory B cells producing RF are believed to drive the formation of these immune complexes. However, we found no evidence HCV patients with cryoglobulinemia (HCV+MC) have increased B cell numbers. To further investigate this phenomenon, we performed a cross-sectional study investigating B cell subsets in (i) HCV patients without MC (n=19), (ii) HCV patients without MC (n=17) and (iii) healthy controls (n=50). While total B cell numbers did not differ among the groups there were dramatic differences in B cell subsets. Specifically, HCV+MC patients displayed 4-fold expansions of activated and immature B cells, a shift from immature transitional T1 to T2 B cells (T1:T2 ratio of 1:5), and reductions in the percentage of nave B cells. Higher immature B cell percentages were also observed in HCV patients, but the normal T1:T2 ratio of 1:3 was not affected. These results suggest that normal B cell numbers in HCV patients with MC are maintained due to apoptosis of nave B cells. In order to characterize the evolution of B cell subsets in cryoglobulinemia we exploited an ongoing study that studied the effects of Rituximab, a B cell depleting antibody, in HCV+MC patients. This drug rapidly depletes all B cells including the pathogenic B cells driving the disease and allowed us to characterize the regeneration of the B cell compartment after Rituximab treatment in HCV+MC patients. Nine HCV patients with MC underwent treatment with Rituximab. Immune reconstitution started 6 months post treatment with almost half of all B cells being immature transitional. Six months later B cell subsets and cryoglobulin levels had normalized. We found that newly generated B cells were more akin to B cells in healthy individuals as evidenced by a normal T1/T2 ratio and reduced percentages of activated B cells. This study provides new insight into the complexity of B cell homeostasis in chronic HCV patients with cryoglobulinemia and explains the mechanism of action of Rituximab.